p2p: Fix not freeing server_data in free_server_data

p2p: Fix not freeing server_data in free_server_data

Free server_data when clearing servers list. This fix following issue
reported by valgrind:

84 bytes in 3 blocks are definitely lost in loss record 167 of 210
   at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
   by 0x4095C5A: ??? (in /lib/i386-linux-gnu/libglib-2.0.so.0.3306.0)
   by 0x40961DC: g_try_malloc0 (in /lib/i386-linux-gnu/libglib-2.0.so.0.3306.0)
   by 0x8056853: p2p_listen (p2p.c:247)
   by 0x407FA21: g_hash_table_foreach (in /lib/i386-linux-gnu/libglib-2.0.so.0.3306.0)
   by 0x805BA2F: __near_adapter_listen (adapter.c:1048)
   by 0x805C64D: near_device_driver_register (device.c:461)
   by 0x80567E2: p2p_init (p2p.c:374)
   by 0x805E317: __near_plugin_init (plugin.c:180)
   by 0x804E0E1: main (main.c:214)